1. Field of the Invention
The present invention relates generally to luminance signal producing circuits, and is directed to an improvement in a luminance signal producing circuit for use in an apparatus in which an image pickup signal obtained from a solid-state image sensor having a color filter array on an image pick-up flat portion thereof is subjected to various signal-processes to produce luminance and chrominance signals constituting a color video signal.
2. Description of the Prior Art
There has been proposed a solid-state image sensor which has an image pickup flat portion comprising a matrix of two-dimensionally arrayed rows and columns of picture units each including a photoelectric transducer formed in a semiconductor substrate and charge transfer domains each comprising, for example, charge coupled devices (CCDs) formed also in the semiconductor substrate for transferring signal charges produced by the picture units. The solid-state image sensor is often employed in a color video camera for producing a color video signal.
In the color video camera in which the solid-state image sensor is employed, a color filter array is provided on the image pickup flat portion of the solid-state image sensor. The color filter array comprises a matrix of two-dimensionally arrayed rows and columns of color filter units corresponding to the picture units in the image pickup flat portion, respectively, and the light from an image impinges through each of the color filter units upon the corresponding one of the picture units.
The color filter units constituting the color filter array includes, for example, red filters, green filters and blue filters. Various positional arrangements of the red, green and blue filters of the color filter array have been proposed. In one of the positional arrangements of the color filter units, the green filters are checkered and the red and blue filters are arrayed in such a manner that a row of the green and red filters aligned one after the other and a row of the blue and green filters aligned one after the other are alternately made. This positional arrangement of the color filter units is called a checkered green-line sequential red and blue arrangement.
In the solid-state image sensor having the image pickup flat portion provided thereon with the color filter array, a signal charge corresponding to a red light component contained in the light from an image is produced to be stored in the picture unit upon which the light from the image impinges through the red filter, a signal charge corresponding to a green light component contained in the light from the image is produced to be stored in the picture unit upon which the light from the image impinges through the green filter, and a signal charge corresponding to a blue light component contained in the light from the image is produced to be stored in the picture unit upon which the light from the image impinges through the blue filter. Then, the signal charges stored in the picture units are read and transferred through the charge transfer domains each comprising the CCDs to an output portion from which an image pickup signal based on the signal charge read from the picture units is obtained. The image pickup signal thus obtained at the output portion contains a red signal component based on the signal charge read from the picture unit corresponding to the read filter, a green signal component based on the signal charge read from the picture unit corresponding to the green filter, and a blue signal component based on the signal charge read from the picture unit corresponding to the blue filter.
When a color video signal is produced based on the image pickup signal obtained from the solid-state image sensor, usually, luminance and chrominance signals which are signal components constituting the color video signal are formed respectively and then synthesized with each other to produce the color video signal. There have been proposed several different kinds of color video signals and one of them is a NTSC color video signal according to the NTSC color television system.
The NTSC color video signal comprises a luminance signal (Y) and a chrominance signal (C) and the chrominance signal (C) contains a red primary color signal (R), a green primary color signal (G) and a blue primary color signal (B). The relation between the luminance signal (Y) and the red, green and blue primary color signals (R, G and B) is expressed by the equation: Y=0.3.multidot.R+0.59.multidot.G+0.11.multidot.B. Accordingly, in the case where a NTSC video signal is intended to be produced based on an image pickup signal obtained from a solid-state image sensor under a condition in which red, green and blue primary color signals corresponding to red, green and blue signal components contained in the image pickup signal, respectively, are obtained, it seems that it would be necessary to apply the above mentioned relation expressed by the equation: Y=0.3 .multidot.R+0.59.multidot.G+0.11.multidot.B to a luminance signal. However, it is not always true in fact.
In practice, low and high frequency components of the luminance signal are separately processed as a low frequency luminance signal (YL) and a high frequency luminance signal (YH), respectively, and it has been proposed, with the intention of improving resolution of pictures reproduced in the strength of the luminance signal, to apply the above mentioned relation to the low frequency luminance signal YL in such a manner as YL=0.3.multidot.R+0.59.multidot.G+0.11.multidot.B and another relation different from the above mentioned relation to the high frequency luminance signal YH in such a manner as, for example, YH=0.25.multidot.R+0.5.multidot.G+0.25.multidot.B.
However, when the relation expressed by the equation: YH=0.25.multidot.R+0.5.multidot.G+0.25.multidot.B is applied to the high frequency luminance signal YH in process of producing the NTSC color video signal based on the image pickup signal obtained from the solid-state image sensor, the following disadvantage would be brought about.
In general, when the color video signal is produced based on the image pickup signal obtained from the solid-state image sensor, each of red, green and blue signal components of the image pickup signal corresponding to a plurality of picture units arrayed in the image pickup flat portion of the solid-state image sensor is obtained through a sampling process performed in response to each row of the picture units. The sampling process through which the red, green and blue signal components are obtained is carried out with a clock signal having a predetermined frequency. Therefore, each of red, green and blue primary color signals which are obtained to correspond to the red, green and blue signal components contained in the image pickup signal, respectively, contains a carrier signal component having a frequency corresponding to the frequency of the clock signal used for the sampling process.
The carrier signal component thus contained in each of the red, green and blue primary color signals is an unnecessary signal component to the luminance and chrominance signals which are produced based on the red, green and blue primary color signals and therefore it is desired for the luminance and chrominance signals to cancel the carrier signal component.
Under such circumstances, in the case where the relation expressed by the equation: YH=0.25.multidot.R+0.5.multidot.G+0.25.multidot.B is applied to the high frequency luminance signal YH, the carrier signal component is not cancelled in the high frequency luminance signal YH due to the values of the coefficients provided to R, G and B in the equation, respectively, so that it is feared that a folding distortion which generally results from a high frequency component intermixing with a low frequency component as a counterfeit signal component is brought about in the luminance signal Y containing the low frequency luminance signal YL and the high frequency luminance signal YH which reproduces a black and white picture. This folding distortion results in deteriorations in resolution of the picture reproduced based on the luminance signal Y.
As described above, it is difficult to achieve hi-fidelity luminance and improved resolution on the reproduced picture by means of the previously proposed signal processing in which the relation expressed by the equation: YL=0.3.multidot.R+0.59.multidot.G+0.11.multidot.B is applied to the low frequency luminance signal YL and the relation expressed by the equation: YH=0.25.multidot.R+0.5.multidot.G+0.25.multidot.B is applied to the high frequency luminance signal YH because the luminance signal Y containing the low frequency luminance signal YL and the high frequency luminance signal YH is adversely affected by the folding distortions.